Laboratory fume hoods are frequently used to exhaust the fumes that are lighter than air and fumes that are heavier than air. When handling lighter than air fumes, it is advantageous to adjust the fume baffle system so that the exhaust pulls air in from an upper portion of the hood while openings through the baffle system to the exhaust in a lower portion of the fume hood are reduced or closed off. Conversely, when handling heavier than air fumes, the inlets to upper portions of baffle systems are reduced or closed off while inlet ports adjacent a lower portion of the fume hood are increased or open. This is sometimes done by angularly moving the entire baffle systems as shown in FIG. 2 of the Grow and Puls U.S. Pat. No. 3,218,953. Other forms of manually moved baffle systems are shown in U.S. Pat. Nos. 1,972,917; 2,627,220; 2,779,265; and 3,022,718. These prior fume hoods are often burdensome to change between lighter than air fumes and heavier than air fumes. Often they require the operator to reach into the hood and make major changes to the baffle system. This was often time consuming because the fume hood had to be completely cleared of any hazardous fumes before the operator would go into the fume hood and alter the physical structure of the baffles, usually which were at the top and back areas of the fume hood.
Other fume hoods have used movable dampers simply to open and shut the exhaust port, often to conserve energy, such as preventing cold air from entering through the exhaust duct work during winter when the building was heated. These movable dampers, such as shown in U.S. Pat. Nos. 1,968,532; 2,590,577; 2,702,505; and 4,177,716, operate in much the same manner as flue vents on stove or fireplace chimneys to open and close the chimney. They were not used in conjunction with baffle systems forming passages at an upper and rear portion of the fume hood for selectively controlling lighter than air and heavier than air fumes.
U.S. Pat. No. 3,897,271 deals with a movable damper-like system for controlling incoming auxiliary air to a fume hood, but does not alter the flow passages of baffle systems for controlling lighter than air and heavier than air fumes.
U.S. Pat. No. 4,177,717 shows the use of rear baffles that can be either fixed or movable relative to a rear wall of the fume hood. As with several of the patents mentioned previously, movement of the baffle itself requires operator entrance into the fume hood.